This special issue contains ten contributions that were originally presented β at the international symposium 'Changing Wetlands' held in Sheffield, UK, between 11 and 13 September 2001, and organized by the Sheffield Wetlands Research Centre (SWeRC) with sponsorship from the British Ecological Society and the Society of Wetland Scientists. The focus of the symposium as a whole was on environmental changes in wetland ecosystems and on opportunities for restoration of wetland systems in terms of their total area and of their floristic and faunal quality. It was clear from many of the papers given at the conference that hydrological processes represent a fundamental component of virtually all wetland ecosystems (the links between hydrology and ecology within wetlands have been discussed elsewhere, e.g. see Wheeler (1999)). Thus, it was thought appropriate to invite authors of some of the more interesting hydrological papers to submit their work for consideration for publication in a special issue of Hydrological Processes.
In the end, ten papers were accepted for publication following peer review. These have been grouped into four broad themes. The first three papers deal with losses of water from wetlands to the atmosphere. Atmospheric losses are often the largest component of water loss in the wetland water budget; yet, characterization of this key loss and elucidation of the effect of wetland vegetation on rates of loss remain problematic. The first paper, by Moro, Domingo and Lopez, describes the novel application of the stem heat-balance method for measuring sap flow, and, therefore, transpiration, from reeds (Phragmites spp.) in a semi-arid wetland. To the editors' knowledge this is the first time that such a method has been applied in a wetland. The research reveals the very high rates of transpiration that are possible in isolated wetlands in a semi-arid setting, where the advection of heat and dry air from surrounding dry land enhances rates of water loss. The papers by Gavin and Agnew and by Peacock and Hess consider how well-established methods of measuring and modelling evaporation and evapotranspiration can be applied to wetlands. Although the methods discussed have been applied widely in non-wetland environments, their use in wetlands has been less routine. The papers provide detailed information on how the Bowen ratio energy balance method and the Penman-Monteith and the Taylor-Priestley models can be applied to wet grasslands and reed beds.
Three papers look at groundwater flow in wetlands. The papers by Clymo and by Baird, Surridge and Money look at the measurement of hydraulic conductivity K of peat soils. Clymo finds that hydraulic properties of a raised bog in southwest Scotland contrast strongly with those reported for a raised bog in Minnesota (Chason and Siegel, 1986), and do not fit the assumptions of Ingram's (1982) widely used groundwater mound model of raised bog growth/shape. Clymo's work clearly suggests a need to consider how hydraulic properties vary between raised bogs in different climatic environments and also to re-evaluate Ingram's standard model. Baird, Surridge and Money use the Piezometer method to measure the K of fen rootmat/peat deposits. They find